Vacuum arc processes have been used in the preparation of pure high melting point metals. A by-product of this process is a thin metal film on the walls of the vacuum chamber. Lucas et al in the publication "A New Deposition Technique for Refractory Metal Films", American Vacuum Society Transactions 2, 1962, p. 988-991, describe a vacuum arc deposition technique to produce refractory metal films, such as noibium, tantalum, vanadium and iron films. This technique used an electric motor or hand lever to mechanically ignite the vacuum arc. The current flowing between an anode and a cathode electrode was interrupted by rapidly withdrawing the movable electrode a short distance from the stationary electrode. The short arc produced between the electrodes yields a vapor flux. This vapor flux condensed on a substrate to produce a metal film.
An electric arc ignited between a cathode and an anode in a vacuum generates cathode spots. Currents of a few amperes to a few hundred amperes yield intense cathode spots and diffuse anode spots. Typically, arcs are sustained in vacuum at anode-cathode potential differences of 10 to 25 volts or more. Arc sustaining voltage is a function of material properties. Substantial power is dissipated in an arc column. For an arc current of 100 amperes and a voltage of 20 volts, power dissipation is 2 KW. In such an arc column, more than half of the arc power is dissipated at the cathode. A large part of the remainder is dissipated at the anode.
Intense cathode spots produced are very small in size. Current density at the cathode spot is of the order of 10.sup.5 to 10.sup.7 amperes per square cm. The arc duration at a cathode spot usually varies between a few to several thousand microseconds. High local temperatures are produced due to high current densities. Cathode spot temperatures approximately equal the boiling point of the material making up the cathode. Due to this high temperature, substantial vapor pressures are developed by vacuum arcs. The vapors produced by cathode spots are used to coat objects placed within the vacuum chamber and to sustain the arc ignited between the cathode and an anode.
Snaper in U.S. Pat. No. 3,625,848 discloses an apparatus for coating an object with a thin film of source material with the use of a vacuum arc. A beam of source atoms and ions is directed at a base or substrate to be coated within a vacuum chamber. The beam gun has a cathode and an anode arranged to sustain an arc discharge. Current flow between the cathode and an igniter electrode attached to a permanent magnet is interrupted to ignite the arc. This is accomplished by passing the current through a solenoidal coil located outside the vacuum chamber. The solenoidal coil field interacts with the permanent magnet connected to the igniter electrode to interrupt current flow in the cathode igniter circuit. This current flow interruption produces a short arc which is sustained between the main anode and cathode. The magnetic field of the solenoid also modifies the operating characteristics of the gun and increases the arc potential between the cathode and anode.
Sablev et al in U.S. Pat. No. 3,793,179 disclose an apparatus for metal evaporation coating. The apparatus embodies a mechanical arc igniter between a disk cathode and a hollow spherical anode. A solenoid armature and an igniter circuit is incorporated in the main arc circuit in such a manner as to automatically re-ignite the arc whenever it extinguishes. The arc once produced is free to move at random at high speed over the cathode surface. Arc spot motion is chaotic and uncontrolled. Periodically, arc spots will pass the edge of the cathode. This extinguishes the arc.
Rapidly moving, short-lived arc spots darting across the cathode surface generate vapor as well as discrete metal particles. Solid metal particles deposited on objects degrade their surface finish and the quality of the coated film. In many applications, such as thin film devices, recording media, optical coatings and tribological coatings, the presence of micron-size solid particles is unacceptable.